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1987 560SEL Astral Silver/Silver blue leather,Michelins,SONY
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820 Posts
Discussion Starter #1
Okay, here is the situation: fuse 15 blows. When I try to replace it, the new fuse blows too.....right away.
Fuse #15 is: antenna, rear inside lights, locking system, trunk light, seat belt light above rear view mirror and maybe couple other things.
Why? What now? A circuit is open, but since I have no electrical testers and no knowledge of automotive electrics is there anything I can do short of a visit to the ($125 an hour) dealers dungeon?
At least I CAN replace fuses! Am getting to be good at it! LOL:)
 

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W124 400E, W126 500SE
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971 Posts
disconnect the circuits in question one by one...

that means pulling the plug on antenna, removing bulbs from the lights, etc...

if it burns again then it's either something you couldn't unplug or harness itself.

you can unplug things under the fuse box if need be, but i'd leave that to an electrician to investigate


Btw I had the trunk light cable break from flexing, it's a common problem, so take a look at that first, maybe there are exposed wires that only touch on bumps or when operating the lid.
 

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Premium Member
2012 CL550 4MATIC Coupe
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9,992 Posts
Fuse 15 has the power seats and a few other items on it. You cannot do any meaningful troubleshooting without a meter, period. You need to be able to check for shorts to ground, and cannot do that by simply unplugging the ant and bulbs. Go ahead and check the antenna, because it can short out. But it could be the power lines leading to it that are shorting to ground, or the seats. Anti theft system and the CLS are also both on there and have intricate power lines. The CLS system uses +12 and ground to tell the pump which direction to pump. You have to have a meter to check ground leaks out.

If you get a meter, I can send you a basic troubleshooting guide to read and I can walk you through some procedures, if you need, before you take it to a mech.
 

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126: 1981 300SD, 1987 300SDL
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119 Posts
I have the exact same problem. I noticed the interior lights were out after installing a new AC Compressor, receiver/drier, hose assembly and expansion valve. I'm wondering if the mist from flushing the evaporator with a funnel instead of a line shorted something.

On my fuse box sheet for fuse #15, it states:
trunk light, roof light front and rear, illuminated vanity mirror, warning monitor term. 5, doorlights automatic antenna
Those definitely don't work (except I don't know what the warning monitor term. 5 is).

I do notice that there is a click in the engine compartment when the driver's door is opened or almost closed that sounds like a relay switch.

I've removed the vanity light visors and dome light.

I do have a multimeter and always get a reading of over 12 volts across the fuse contacts before and after removing those parts.

How would I use a multimeter to use a process of elimination of which component is faulty?

Would I check for a drop in amperage after removing each component (e.g. dome light)?

I'm assuming I could try a new fuse every time. heh.

What is warning monitor term. 5?

Is this something related to the, "warning module"?

Maybe I misted the module and gave it a short or a straw on the back of the camel inside. I suppose this might be an excuse to remove it and the "cruise control amplifier" module to clean both and maybe touch up solder.

Why am I lucky enough to have awesome people like you reading these posts?

mclare, Is there any chance i can get my hands on the troubleshooting guide you're talking about?

Thanks!
 

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1987 560SEL Astral Silver/Silver blue leather,Michelins,SONY
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820 Posts
Discussion Starter #5
Thanks to Mclare and others for thoughts. Probed around yesterday and concluded this is simply beyond my abilities and will have to wait to get it to dealership. There are too many things on that fuse that I cannot access. I guess I CAN get along without the radio and the rear lights and the vanity mirror lights! Ha Ha! At least for a few weeks.
When I eventually find out the probem I will post it.
 

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1987 560SEL Astral Silver/Silver blue leather,Michelins,SONY
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820 Posts
Discussion Starter #6
Quick follow up: when you fellas talk about unplugging the antenna, just WHERE is it that I unplug it. At least that is one simple thing I can do and that might be the problem, as I have had antenna troubles on Benzs' in years past.
Otherwise I'll just drive it the way it is. There is no fuse in #15 bay now of course, since they pop as soon as you put them in.
Ahhhh.....yes, always something isn't it! Ha Ha ha!
 

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126: 1981 300SD, 1987 300SDL
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119 Posts
You may have to remove some clips, but behind the left side covering in the trunk, you should be able to see the antenna motor, wiring and sunroof and its wiring. If you following the wire from the antenna motor, you can unplug it. It only goes back one way so you don't have to worry about putting it back wrong.

I've also seen a post on another website where someone found their front dome light was shorted on the circuit board for the timer delay and something about a relay.

"Now that you have the circuit board out you need to test it. It's mostly resisters; there's a capaciter, a solid state relay and a 1455 timer chip, but that's it. In my case the relays was fried and shorted 12V to ground. Nice. So, If I'd been a bit more careful taking it apart I'd be replacing a $2 part right now instead of having to get a hole new unit."

So far, causes could be, antenna motor, dome light, visors, maybe warning module. in my case, maybe the module. I should unplug it and see. It's under the driver's dash if you remove the bottom cover with a few screws. It's on the right of the bigger cruise control amplifier. Maybe I should just start risking some fuses...
 

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1986/1990 W126
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14,102 Posts
That click under the bonnet is likely your comfort relay, which allows the windows to be operated without the key in ignition, when front doors are ajar.
 

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126: 1981 300SD, 1987 300SDL
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119 Posts
On what year and model vehicle do you find this, "comfort relay"? I don't think it's on my '81. There is no such thing in my electrical diagrams and the windows do not behave that way when the driver's door is moved. The driver's door is the only one that results in a click under the hood.
 

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Proud owner of an Blue-Black (Charcol Gray) '91 420SEL
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704 Posts
The door switch for the light is very easy to pull out of the jamb and you may have a wire that has fallen off.
A small flat bladed screwdriver will get this out for you and there are only two wires there. If the fuse pops when you open the driver's door then you are miles ahead in troubleshooting.
If this is the cause then you can temporarily stow the wires for this switch and use the car normally until such time you can either repair it yourself or have it repaired.
Be sure to insulate the wire terminals seperately with some good quality electrical tape or similar item and then tie them to the switch and reinstall it.
Then you can get to the wires down to the road as they will not have fallen down into the cavity forward of the door jamb.
IF you have one more spare fuse that you can sacrifice then with the wires clear of the sheetmetal short them together and see if the light works and and to see if the fuse does not blow. This can tell you if you have a localized problem or if the problem is deeper into the harness and also if the switch is somehow shorting to ground as well. If all works correctly then you have a very local to switch problem. Just my 1.1 cents worth.
The switch is about $8.00 US through AutohouseAZ if you find that it is bad.

jimm1009
 

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1967 W113 250SL, 1999 W210 E300, W126 420SEL, 1986 e28 535i
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47 Posts
Oh wow what a coincidence, had to deal with #15nthe other day. Any chance you are attempting to ground something? I was installing a new Hirschmann telescoping antenna and was silly enoughnto forget to unplug the battery. Then forgotnagain after i figured it out and blew it again.

Here's my thread, where you may be able to get some helpful info, even though most might be unrelated, unless you're replacing an antenna.

http://www.benzworld.org/forums/w126-s-se-sec-sel-sd/1666606-antenna-install-problems.html

Has some great help from Dr.Grillz as well as MBLuv. Hope it helps you in some way.
 

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1987 560SEL Astral Silver/Silver blue leather,Michelins,SONY
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820 Posts
Discussion Starter #12
Okay, here's the morning report on Fuse Fifteen! Was able to find the antenna wiring, just where one of you guys said it would be. Simply pulled a little blue and a little red wire. There was another larger (pencil dia) cable laying in there with a "spike" ferrule in the end. I found a similar cable with a female receptor but it was not compatible with the spike end I found. My guess is that those cables were the original and the small blue and red wires and their little plug in terminal was somebody's replacement.
At any rate, it did nothing for the instant blowing Fuse #15 so I plugged them back together as they were.
I found a dead bulb in the trunk lid, but I am sure has been dead for some time. It is a goofy looking bulb, elongated like a watermelon. My guess is that I cannot buy it at any of the part stores and it will be found only at M-Benz where the price will be $695.75 per bulb plus shipping.
I can drive the car of course. But how do I get at the seat belt alert lights up by the r-view mirror. If I can get in there and pull that bulb I'll be fine until I can afford to go to a dealer for a serious crack at my fuse problem. I'm guessing $125 and hour at dealer and I don't want to have hours and hours spent findiing this problem.
Things like this are one of the reasons I was hesitant to get back into Benzs' again. Not the repair and fixing of stuff, it is the blatant raping that does me in. LOL
 

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1986/1990 W126
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14,102 Posts
The bulb for the boot light is just a standard type that can be found anywhere. Its called a festoon bulb over here, maybe there too. Just take it with you into any motor type shop.

The lights by the seatbelt alert thing, that whole dome light assembly just pulls out downwards, you have to do one side first though and I cant remember which side but its got a kind of flat metal spring holding it in.. The other side kind of hooks in. The bulbs for that thing are just little things. Dont go to the dealer, just ask us!

When you take out that dome light thing, remember when you are popping it back in, to make sure that rubber tube you will see if still connected, and if not then stick it back on. Its the sampling tube for the climate control. Dont worry its super easy.
 

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1986/1990 W126
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14,102 Posts
On what year and model vehicle do you find this, "comfort relay"? I don't think it's on my '81. There is no such thing in my electrical diagrams and the windows do not behave that way when the driver's door is moved. The driver's door is the only one that results in a click under the hood.
Dont know, sorry. I assumed they all had this.
 

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1981 300 SD, 1991 Geo Metro 4-door Automatic
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487 Posts
FWIW...

When I bought my hooptie, I had a similar problem...no windshield wipers, fuse 7 IIRC. What I did:

Bought a pack of fuses (plastic Autozone, but they seem to work fine for me. YMMV)
When new fuse blew, lifted fuse block to access wires. Used meter to find power feed for that fuse. Removed all wires from opposite side of fuse. Checked for shorts. Found that wiring for RH remote mirror control was shorted (zero ohms). Other wires seemed OK (resistance of from a couple ohms to infinity between those wires and ground). Put those wires back on the fuse, I had wipers again. I did this during a nice summer thundershower, which here in Lancaster usually is a few drops of rain here and there, with occasional downpour.

What you need: A meter (a cheapie from Harbor Freight or RadioShack will do)
A wiring diagram...at that time, all I had was a Chilton's, the factory diagram is better, but the Chilton's worked.
A bit of patience and perseverance to work in that rat's nest under the fuse box.
When checking wires, zero ohms indicates a short. Anything above zero ohms most likely indicates a good circuit, depending on what that circuit is. Check with switches for the listed items both on and off, if possible.

Hope this helps (if you wish to tackle this problem yourself and save a few bucks), it's not a complete tutorial, but it may get things working again.
 

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Premium Member
2012 CL550 4MATIC Coupe
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9,992 Posts
I have a Benz troubleshooting guide if anyone wants it, PM me.

Yes, get a Meter from Harbor Freight. The big one is the one I have with the big readout and remote probes.

Sourdough, if you like, if you want to try and after you get the meter, call me and I can walk you through how the wiring diagrams go and maybe you can get some results. You don't have to be an Electrical Engineer to read the schematics. It helps, but you don't need to be. If you can read a map (I mean really read a map, not GPS) then you can read a Mercedes schematic. They are easy to read, and all color coded.

There's not much on that fuse, 5 items on your car. Courtesy Lamps, Power Seats, Anti-theft Alarm system, Central Locking system, which is tied into the Alarm, and my guess as to what may be the issue here. And the antenna.

One other thing it could be is the puddle lamps. They can ground against the metal of the doors.

Do you have the ETM that I sent you? Get it out and look at page 105/5 and you will see fuse 15 there and you will see where all the power taps go and you just have to trace them out one by one. Look, it takes time, but with some basic instruction, you can give it a shot? What would you be out? $30.00 for the meter?

Looking at the schematic, connector "X18" Pin "1" takes the feed from the fuse block +12VDC and that is just before the first split. X18 is a large 12 Pin Molex connector under the driver's footwell, behind the lower dash panel next to the parking brake. Pin one comes directly from fuse 15. Disconn that and then see if you get a trip. Then plug it back in and work your way back. All you have to do is trace the harness thru the car and disconn one by one the systems until you find it.

It's not magic, just patience and a little detective work.
 

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1987 560SEL Astral Silver/Silver blue leather,Michelins,SONY
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820 Posts
Discussion Starter #17
Thanks Mclare. I just might get a meter tomorrow and play around a bit. I tend to agree with you that the problem is going to be found in the anti-theft/door locking system. As it is I can drive the car of course and take some reasonable time getting the problem resolved.
I DO have all the materials you sent me so assume I have the "ETM". Will check it out in a few minutes.
Thanks again. And to all who are participating.
 

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Premium Member
2012 CL550 4MATIC Coupe
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9,992 Posts
Thanks Mclare. I just might get a meter tomorrow and play around a bit. I tend to agree with you that the problem is going to be found in the anti-theft/door locking system. As it is I can drive the car of course and take some reasonable time getting the problem resolved.
I DO have all the materials you sent me so assume I have the "ETM". Will check it out in a few minutes.
Thanks again. And to all who are participating.
Take the alarm unit out and see it that's it. It's under the passenger side footwell. Then try disconnecting the air pump in the trunk in the pare well.
 

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126: 1981 300SD, 1987 300SDL
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I suppose one could unplug everything and put in a fuse and if it doesn't blow, you've isolated something somewhere. Then it's a matter of plugging things in one by one and checking the fuse every time. Whatever blows it is the culprit. Of course if you unplug everything you can which I assume doesn't include the seats and it still blows, then it's either whatever you couldn't unplug, or a wire/connection between the fuse and hot connection to some component that was unplugged, that has shorted to ground.

In my old 1981 case, there are fewer things, but I'm very suspicious of that warning module (casts an evil eye on it).
 

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Premium Member
2012 CL550 4MATIC Coupe
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9,992 Posts
I suppose one could unplug everything and put in a fuse and if it doesn't blow, you've isolated something somewhere. Then it's a matter of plugging things in one by one and checking the fuse every time. Whatever blows it is the culprit. Of course if you unplug everything you can which I assume doesn't include the seats and it still blows, then it's either whatever you couldn't unplug, or a wire/connection between the fuse and hot connection to some component that was unplugged, that has shorted to ground.

In my old 1981 case, there are fewer things, but I'm very suspicious of that warning module (casts an evil eye on it).
That's exactly the course I wanted him to take since his troubleshooting skills are limited. Chances are good that a module shorted somewhere, so that's the easy T/S method.
 
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